The present invention relates to a method for pickling steels and, more specifically , carbon steels, magnetic steels (containing Si) and stainless steels with a chrome content lesser or equal to 15% (i.e. AISI 409), wherein the Fe3+ ion is used as an additive in the bath to increase the reaction kinetics.
In order to realise an efficient pickling process of the carbon steels, class whereto also magnetic steels containing Si belong, hydrochloric (HCl) or sulphuric (H2SO4) acid is normally used, or mixtures thereof, at a temperature that generally varies between 60 and 75xc2x0 C. For stainless steels with a low chrome level, e.g. AISI 409 of the ferric class, analogous baths can be adopted as well.
The main reaction of pickling, to allow the removal of the scale of thermal oxide, is the dissolution (through oxidation) of the iron base according to the anodic half-reaction:
Fexe2x86x92Fe2++2e.xe2x80x83xe2x80x83(1)
The corresponding cathodic half-reaction in acid environment is the ion H+reduction that develops hydrogen:
2H++2exe2x86x92H2xe2x80x83xe2x80x83(2)
Therefore, the resulting total reaction is:
Fe+2H+xe2x86x92Fe2++H2xe2x80x83xe2x80x83(3)
As is known from the study of electro-chemical reactions, the kinetics thereof is strongly influenced by the values of the electrode potential.
An additive of oxidising species is added to the pickling bath, enabling to obtain a more noble electrode potential of the steel to be pickled, thereby allowing higher reaction kinetics.
The species preferred as oxidant is the Fe3+ ion.
The possibility of obtaining a more noble potential of the steel to be pickled derives from the fact that the added ferric ions Fe3+ behave as oxidant (being reduced to Fe2+ ions) with respect of the steel (Fe) that is to be pickled according to reaction (1) (that derives from the reversible electrochemical potential scale: Erev=xe2x88x92447 mV SHE for the Fe2+/Fe couple; Erev=+771 mV SHE for the Fe3+/ Fe2+ couple).
Therefore, the Fe3+ ion is capable of being reduced to Fe2+ ion during the pickling process, according to the cathodic half-reaction below:
Fe3+xe2x86x92Fe2++exe2x80x83xe2x80x83(4)
Also the cathodic half-reaction (4) occurs at the same time of the anodic half-reaction (1). The resulting reaction (1)+(4) therefore is:
Fe+2Fe3+xe2x86x923Fe2+xe2x80x83xe2x80x83(5)
When the Fe3+ ion is present as an additive the reactions (3) and (5) occur at the same time, with an increase of the global iron (Fe) dissolution kinetics.
The working electrode potential Ew results, in this case by effect of the addition of the oxidising species (Fe3+) in the bath, more noble than the potential in absence of additions.
The amount of Fe3+ ion that has to be added must balance the amount of Fe3+ ion consumed in the cathodic reaction(4).
A good stirring of the bath further improves the pickling kinetics, allowing the depolarisation of the total cathodic reaction resulting from the sum of the reactions (2) and (4) that would tend, without stirring, towards diffusion control conditions.
In EP 0792949A1 a pickling process for steel products is disclosed in which the concentration of Fe+3 ions is comprised in the range 1-300 g/l by oxidation of the Fe+2 ions obtained during the pickling.
According to the present invention, the Fe3+ ion can after all be added directly, e.g. as ferric chloride or ferric sulphate. However, it can be obtained in the bath by the addition of other oxidants, particularly H2O2 (hydrogen peroxide), ozone or permanganates. In fact, once added to the pickling bath those oxidants come to contact with a solution already rich in Fe2+ (due to the primary pickling reaction (1) the bath is always rich in Fe2+ ions) and induce the oxidation of Fe2+ to Fe3+. Even oxygen alone, added in conditions of stirring by air bubbling, or admixed in an external reactor with the pickling solution, works as an oxidising agent capable of producing Fe3+.
A preferred embodiment of the method described here according to the invention is that of obtaining Fe3+electro-chemically, sending the pickling solution as anolyte in an electrolytic cell, and carrying out an anodic oxidation of the Fe2+ ion that, as aforestated, is always present in the bath, according to the reaction:
Fe2+xe2x86x92Fe3++e.xe2x80x83xe2x80x83(6)
Therefore, it is an object of the present invention to provide a method for pickling steel products, wherein in the pickling bath Fe3+ is present in a concentration comprised in the range 6-60 g/l, directly added or produced in the pickling bath itself by the addition of an oxidising agent selected from the group comprising: hydrogen peroxide, ozone, permanganates, persulphates, and oxygen.
Advantageously, the obtained increase of the pickling kinetics is a function of the added Fe3+ concentration that is maintained in the pickling bath itself, thereby improving also the productivity of the industrial lines.
A further advantage, according to the invention, lies in the fact that the maintenance and/or the control of the Fe3+ ions concentration in the pickling bath allows a strict control of the most critical parameter of the process (that is the potential redox value of the system) with further advantages on the final quality of the product as well.
According to the present invention, continuous pickling lines of carbon and/or magnetic steels can be employed advantageously also for the pickling of stainless steels with a  less than 15% Cr content. This result was made possible by the fact that, according to the present invention, the Fe3+/Fe2+ ratio is employed as a control parameter of the reaction kinetics, jointly with the acid concentration control. In fact, when it is desired to use the same carbon and magnetic steels production line for the stainless steels, e.g. of the AISI 409 type, the presence and the maintenance of appropriate Fe3+ concentration values allows in any case to widely control the pickling kinetics, for the carbon steels as well as for the stainless ones, thereby making possible the combined utilisation of these production lines.
Advantageously, the method according to the invention proves to be compatible with the possible presence of corrosion inhibitors, normally employed to avoid drawbacks of carbon steels over-pickling.
Furthermore, always according to the present invention, it is possible to pickle these abovementioned steel types without resorting to mechanical processes of descaling such for instance as peening.
A further object of the present invention is to provide different embodiments, as hereinafter specified, of the method for the direct production of the oxidant Fe3+ within the pickling bath:
(i) direct addition of Fe3+ as reactant (for instance: ferric chloride or sulphate);
(ii) addition of oxidants for the production Fe3+ by oxidation of the Fe2+ ion present in the bath to Fe3+;
(iii) oxidation of the Fe2+ ion in an electrolytic cell present in the bath itself to Fe3+.
According to the third embodiment, the same pickling solution itself (composed of aqueous solution of hydrochloric acid and/or sulphuric and, optionally of phosphoric acid) constitutes the cell anolyte, the oxidising agent to be added as additive being the ferric ion produced at the anode by oxidation of the ferrous ion present within the bath.
The catholyte is preferably composed of an aqueous solution of hydrochloric and/or sulphuric acid. Also the catholyte is preferably sent out continuously into the pickling solution, to reintegrate the HCl or the H2SO4 that is consumed during the pickling reaction.
According to the method of the invention, an electro-chemical cell of the membrane type is preferably employed as shown in FIG. 1.
The use of a cell to generate the oxidising species Fe3+, according to the third embodiment, is more advantageous in respect of a method wherein oxidants are directly added. A remarkable saving in operation cost is obtained, on account of the higher cost of the oxidising reactants themselves.
Furthermore, some reactants entail stability problems within the bath, or may cause, if not opportunely measured, the development of chlorine from the hydrochloric acid bath.
The pickling bath according to the present invention has a temperature preferably comprised in the range 45-85xc2x0 C.
The pickling solution is an aqueous solution of hydrochloric and/or sulphuric acid and optionally of phosphoric acid, with the composition hereinafter expressed as percent by weight:
free HCl from 0 to 250 g/l ( greater than 100 g/l if alone);
free H2SO4 from 0 to 250 g/l ( greater than 100 g/l if alone);
free H3PO4 from 0 to 100 g/l;
Fetot=(Fe2++Fe3+) greater than 50 g/l;
Fe3+ (additive) from 5 to 60 g/l;
Furthermore, products containing iron (steels) whereto the method of the present invention can be applied are selected from the group comprising:
carbon steels, rolled or anyhow hot or cold worked, Particularly low carbon steels and carbon steels with a low, medium or high content of alloying elements;
magnetic steels (containing Si or Si and Al);
stainless steels with a low ( less than 15%) Cr content, as, particularly, AISI 409.
The present invention will be more clearly illustrated in the following detailed description of a preferred embodiment thereof, given as a non limiting example, with reference to the annexed figure.
The addition of the Fe3+ ion as oxidant if performed according to the methods as per the embodiments (i) and (ii) is carried out by the introduction of the reagent into the bath in stechiometrically calculated amounts and considering the yields, both automatically and manually.
According to the invention, in the method of the embodiment (iii), an electro-chemical cell is employed.
According to this method an electro-chemical treatment of the solution is carried out, whereby it is directly obtained in situ formation and the control at the appropriate concentration levels of the oxidising species Fe3+, originated by the Fe2+ species anyhow contained within the bath.
The control of the Fe3+ ions concentration within the pickling bath and/or of the Fe3+/Fe2+ ratio is obtained in an easy way by the setting and the regulations of the operative parameters of the cell.
Hereinafter, the principles and criteria for the construction of a Fe3+ producing electro-chemical cell are defined.
a) Anolyte
The pickling solution itself is employed, continuously circulated (but a discontinuous treatment as well can be foreseen) from the bath by pumping;
b) Anodic Reaction
The anodic half-reaction which occurs in the cell is:
Fe2+xe2x86x92Fe3+e.xe2x80x83xe2x80x83(6)
Regulating the cell flow the reaction (6) kinetics is controlled, and it becomes therefore possible to keep steady at the desired level the concentration of the oxidising additive Fe3+ in the pickling solution.
c) Catholyte
It was found that the most convenient way consists in utilising as catholyte a hydrochloric and/or sulphuric acid solution, that is sent to the bath according to the Fact that the specific pickling process foresees the in-bath utilisation of hydrochloric or sulphuric acid or mixtures thereof. In principle, however, any catholyte whatsoever may be utilised, or even directly the pickling solution, if the catholyte in this case is sorted out as exhausted.
d) Cathodic Reaction
The cathodic reaction, for ease of description referred only to the hydrochloric acid, is:
2H++2exe2x86x92H2(cathodic half-reaction)xe2x80x83xe2x80x83(2)
e) Anodic Control
Regarding the control of the anodic current flow inside the electrolytic cell two alternatives are effective:
e.1) Potenziostatic Cell Control
Operating with an electrode potential( greater than 771 mV SHE) that allows the oxidation reaction (6); regarding the maximum value it is advisable to choose a maximum value that does not allow (or anyhow limits to values that are not excessive) oxygen development, according to the following reaction:
O2+4H++4exe2x86x922H2O (Erev=+1229 mv SHE)xe2x80x83xe2x80x83(7)
Theoretically, the preselected potential E finally lies within the range 771-1229 mV SHE. In practice it can prove useful to set it at values even relatively higher than 1229 mV (e.g. 1600), exploiting the fact that the oxygen development reaction occurs at a certain overvoltage and the kinetics involved may be negligible.
e.2) Galvanostatic Control
This control is simpler (more cost-effective) to realise in a plant, but the abovementioned advantages might be lost.
f) Membranes
Different commercial membranes may be used, that differ in efficiency, employ temperature, duration, dimension.
The electrochemical cell considered, tested in a pilot plant, gave the following performances, that are reported hereinafter, merely by way of exemplification:
current efficiency: greater than 95%
cell potential (xcex94V at terminals)≅2V
specific power≅b 5W/dm2 
anodic current density≅5A/dm2 
consumption per mole of Fe3+ produced  less than 0.1 kWh
Subsequently, preliminary lab tests were carried out, demonstrating that higher than standard weight losses are obtained by employing Fe3+ additioned solutions. Finally, experimental results were verified in industrial processes.
Further, the method of the invention is applicable for pickling products made of titanium and alloys thereof.